If you go back in time you will find that you have a lot of ancestors, 2 for one generation back, 4 for 2 generations back, 8 for 3 generations back and so on. Obviously this isn't accurate anymore if you go back like 30 generations because it would mean you have 2^30 or about 1.07 billion ancestors, which is more than the total human population back then. So how much time approximately do you have to go back to find a common ancestor for the majority of our planets human population?
According to a 2003 study from MIT, the most common ancestor for the majority of humanity lived in Asia between 2000 to 5000 years ago. According to the same study, nearly everyone living a few thousands of years prior to that was either the ancestor of no one, or all living humans.
You may also be interested in our most recent patrilineal (the father of your father's father's father's father's.... father) and matrilineal (mother of your mother's mother's mother's mother's... mother) ancestors:
"Eurasian Adam" lived around 68,500 thousand years ago (~2400 generations) in Northern Africa, and is the most recent common patrilineal ancestor of Haplogroup CT which includes all people outside of Africa as well as most Africans.
The most common patrilineal ancestor of all humans living today is called "Y-Chromosome Adam", and he lived between 120,000 and 338,000 years ago. The most common matrilineal ancestor ("Mitochondrial Eve") lived between 99,000 and 200,000 years ago.